1. Field of the Invention
This invention relates to a nonvolatile memory element, and more particularly to techniques for storing an information by utilizing hysteresis characteristics of ferroelectrics.
2. Description of the Prior Art
As nonvolatile memories, there have been proposed FETs (field effect transistors) using a ferroelectric film. An example of FETs using such a ferroelectric film (e.g., films made of PZT (PbZr.sub.x Ti.sub.1-x O.sub.3)) is shown in FIG. 10. The FET 12 shown in FIG. 10 is called an FET of an MFMIS type (Metal Ferroelectric Metal Insulator Silicon), and includes a semiconductor substrate 2 having a channel forming region CH on which a gate oxide film 4, a floating gate 6, a ferroelectric film 8 and a control gate 10 in turn are formed in this order.
When the substrate 2 of the FET (N-channel) is connected to ground and a positive voltage +V is applied to the control gate 10, the ferroelectric film 8 undergoes polarization reversal. Even when the voltage applied to the control gate 10 is released, a negative charge is generated in the channel forming region CH due to remanent polarization of the ferroelectric film 8. This state is set as "1".
On the contrary, when a negative voltage -V is applied to the control gate 10, the ferroelectric film is allowed to undergo polarization reversal in the direction opposite to that in which polarization reversal occurs due to the application of positive voltage +V. Even when the negative voltage applied to the control gate 10 is released, a positive charge is generated in the channel forming region CH due to remanent polarization of the ferroelectric film 8. This state is set as "0". In such a manner, the information ("1" or "0") is written in FET 12.
In order to read the written information, a read voltage Vr is applied to the control gate 10. The read voltage Vr is set to a value between a threshold voltage Vth1 of the FET in "1" state and a threshold voltage Vth0 of the FET in "0" state. Accordingly, when the read voltage Vr is applied to the control gate 10, the written information "1" or "0" can be recognized by determining whether or not a predetermined drain current is detected. Upon the read operation, the written information is not erased.
Thus, the FET using a ferroelectric film enables so-called nondestructive read. For this reason, in such an FET, the stored content can be prevented from being destroyed at each time of read operations. This ensures a high operating speed upon the read operation, and a low power consumption. Further, since the ferroelectric film shows less deterioration, the stored content can be retained with a relatively high reliability.
However, the above-mentioned FET using the ferroelectric film has the following problems. As shown in FIG. 11, upon write operation, the FET 12 has such a configuration that a capacitor Cf (capacitance Cf) including the ferroelectric film 8 and a capacitor C0 (capacitance C0) including the gate oxide film 4 are connected in series to each other. Therefore, in the case where the voltage V (=+V or -V) is applied between the substrate 2 and the control gate 10, the capacitor Cf including the ferroelectric film 8 is applied with a divided voltage Vf represented by the following equation: EQU Vf=C0/(Cf+C0).multidot.V
When it is intended to cause polarization reversal of the ferroelectric film 8 upon write operation, it is necessary to increase the above-mentioned divided voltage to some extent. To this end, as is recognized from the above-mentioned equation, it is required to reduce the capacitance of the capacitor Cf relative to that of the capacitor C0 to some extent. However, the relative dielectric constant .epsilon.f (200 to 1,000) of PZT from which the ferroelectric film 8 is formed, is considerably large as compared to the relative dielectric constant .epsilon.0 (3.9) of SiO.sub.2 from which the gate oxide film 4 is formed.
For this reason, if both the thickness t.sub.0 of the gate oxide film 4 and the thickness t.sub.f of the ferroelectric film 8 are kept constant, it is required that the gate oxide film 8 has a considerably large area as compared to that of the ferroelectric film 8, in order to increase the divided voltage Vf according to the above-mentioned equation. This inhibits intended miniaturization of the nonvolatile memory.
On the other hand, the divided voltage Vf according to the above-mentioned equation can also be increased by decreasing the dielectric constant of the ferroelectric film 8 up to approximately a similar level to that of the gate oxide film 4. However, it is not easy to reduce the dielectric constant of the ferroelectric film 8. In order to reduce the dielectric constant of the ferroelectric film 8 to a level similar to that of the gate oxide film 4, a material of the ferroelectric film 8 must be limited to particular ones. However, in the case where such particular materials are used, it becomes difficult to produce a ferroelectric film having desired properties.
Further, when the read voltage Vr is applied to the control gate 10 (refer to FIG. 10) in read operation, the polarization reversal of the ferroelectric film 8 is not normally caused. However, while such a read operation is repeated many times, the remanent polarization of the ferroelectric film 8 is gradually decreased, so that read error is likely to occur.